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Let’s just get the alliteration out of the way in the first paragraph: solar panels are the perfect way to power your pool pump. Running a 1-horsepower pump for an average of 5 hours per day requires about 7.25 kilowatt-hours (kWh) of electricity. That can cost between $1 and $2 per day, depending on where you live.

That might not sound like a whole lot—you probably pay more for coffee every day—but it can add up to $300 to $600 per year. And since a solar panel system will last at least 25 years, we’re talking $15,000 or more in potential electricity savings. But first we have to find out how much the solar system costs, to see whether it’s worth your money.

To do this, we need to take three steps:

Find out how much energy you can get from a 1-kW solar system in your area

Determine how big of a system you’d need to provide all the energy for the pool pump and how much it will cost.

Plot the initial cost and annual savings on a 25-year chart to see if the panels will pay their cost back in time.

Step 1: Find out how much energy you can get from 1 kW of solar panels:

The map below shows the number of kWh you can expect to generate for every kilowatt of solar panels you have. Find your number, then multiply by 0.78 to account for energy losses due to wiring, DC/AC inversion, temperature and other factors.

Here’s a quick table of what you can expect in certain parts of the country where you’re likely to find a lot of backyard pools:

Step 2: Determine the size and cost:

The 7.25 kWh we need for the pump every day is about 2,650 kWh per year. To calculate the system size you need, divide 2,650 kWh by the number for a 1-kW system from the last step. The result is the system size, measured in kW.

For example: In Florida, you divide 2,650 kWh (your needs) by 1,326 (yearly production of 1 kW of solar), equals 1.998491704374057… uh, 2. So you need a 2-kW solar panel system.

Finally, we’ll multiply the system size by $3,500, which is the average upfront cost per kW of solar in the U.S. (as of mid 2016), and then multiply that by 70%, to account for the reduction in the first-year cost to you from the federal 30% solar tax credit.

Step 3: Plot the cost and savings on a graph:

This is the step where you get to see how well your investment pays itself back, but it isn’t as simple as adding up a grocery bill. You need to account for a few factors, including increases in electricity prices and reductions in efficiency of your solar panels over time.

With that in mind, you might actually want to bump up your system size so that the production will equal your pump’s needs over the long run, but the difference isn’t a huge amount, and the decrease in production often isn’t as high as 0.5% per year.

So the numbers are run as follows: 1st year equals the cost of the panels minus the tax credit and energy savings (production multiplied by cost), and each subsequent year’s energy savings added until year 25. Let’s do an example for Florida, which has an average electricity price of $.12/kWh and an average production of 2,652 kWh in year 1. With those numbers, you come up with this graph:

Not too shabby! Your electric bill savings pay the initial cost of the panels back in year 13, and you keep getting the free solar energy until at least year 25, guaranteed by the panel manufacturer. That results in a total profit of about $6,700, with an internal rate of return (IRR) of 7.69%. Pretty good!

But California, is a whole ‘nother story, because the state gets more sun (which means a smaller system), and the electricity prices there are through the roof! With a $.17/kWh average price, 2,964 kWh average 1st-year production, here’s what the graph looks like:

Yowza! That’s a 9-year payback, with a total profit of $12,086 and an IRR of 13.7%. No wonder Cali has the most residential solar in the country. Those savings are huge!

An the best part is, even if you decide you hate your pool and want to fill it in with concrete, the solar power can still be used to power your home, or just sent back onto the grid (in net metering states) and offset your electric bill. Badabing, badabang, badaboom, you’re a smart solar haver!

What about heating the pool?

We’re not experts in the solar water heating field, but we know the limitations of our beloved solar electricity, and we know when it’s right to choose something else. According to Florida Power & Light, you need about 1,000 kWh (depending on where in Florida you live) to run a heat pump water heater for a 14’x28′ pool.

That’s a heck of a lot of electricity (it would cost $120 per month!), and even though getting that power from solar panels would be cheaper, there is a better way: a solar pool heater.

Basically, we’re talking about a series of tubes made of black plastic or rubber, or sometimes copper under glass, that are placed somewhere sunny. Water is pumped through the tubes, taking on a ton of heat just by being in a sunny spot. Here’s how the whole thing works:

Now, that “gas heater” can be a heat pump or whatever you currently use to heat your pool water, but the gist is, it’s only useful on days when it’s too cloudy or cold to use the solar collectors alone. The collectors still add thermal energy (heat) to water, they just sometimes need a little help from a separate heater. But in places like Florida, Southern California, and Arizona, your solar water heating system probably won’t need too much help.

The financial impact of a solar water heating system

Let’s add the cost and savings of a solar water heating system to our earlier estimates. Here are the assumptions we’ll use for these calculations:

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